Graphite is a crystalline form of carbon comprising atoms bonded in flat layered planes with weaker bonds between the planes. By treating particles of graphite, such as natural graphite flake, with an intercalant of, e.g., a solution of sulfuric and nitric acid, the crystal structure of the graphite reacts to form a compound of graphite and the intercalant. The treated particles of graphite are hereafter referred to as intercalated graphite flake. Upon exposure to elevated temperatures the particles of intercalated graphite expand in dimension in an accordion-like fashion in the c-direction, i.e. in the direction perpendicular to the crystalline planes of the graphite.
Intercalated graphite flake has many useful applications. A common application is to exfoliate the intercalated graphite particles into vermicular-like structures which are then compressed into sheets of flexible graphite for use in the manufacture of gaskets or as packing material. Intercalated graphite flake is also used in a variety of products which take advantage of the high expansion characteristic of intercalated graphite flake when exposed to high temperature. One such example is for use in combination with polymer foams to form seat cushions and furniture upholstery in aircraft. Upon exposure to fire, the high temperature will cause the particles of intercalated graphite to exfoliate which minimizes or prevents the formation of toxic gases from the polymer foam and may, of itself, smother a fire.
Since it is important to suppress, i.e. retard a fire before it has begun to spread, it would be a substantial advantage for an intercalated graphite flake product to exhibit a very high degree of exfoliation upon exposure to temperatures as low as 600.degree. C.
It has been discovered in accordance with the present invention that the treatment of intercalated graphite flake with an organic reducing agent, following intercalation of the graphite flake with an oxidizing intercalant solution, and while the graphite flake is covered with a coating of oxidizing intercalant solution, results in a material which exhibits enhanced exfoliation volumes at exfoliation temperatures as low as 600.degree. C.